1. Technical Field
The present invention relates to a process for recovering liquid hydrocarbons from a subterranean hydrocarbon-bearing formation, and more particularly to such a process wherein an emulsion having a supercritical carbon dioxide phase and a liquid aqueous phase is placed in the formation to enhance liquid hydrocarbon recovery therefrom.
2. Description of Related Art
Carbon dioxide is a particularly attractive oil displacement fluid because of its relatively plentiful low-cost availability and because of its relatively high degree of solubility in oil, either at elevated pressures by means of a first contact mechanism, or at lower pressures by means of a multiple contact mechanism. Nevertheless, the effectiveness of carbon dioxide flooding is often offset by the high mobility of carbon dioxide due to its low viscosity, whether in a low-pressure gaseous state or in a high-pressure supercritical state.
One promising method for increasing the viscosity of carbon dioxide floods, thereby decreasing the mobility of the flood, is described in Wellington et al., "CT Studies of Surfactant-induced CO.sub.2 Mobility Control", SPE Paper No. 14393. Sep. 1985. As taught therein, a carbon dioxide gas is dispersed in a brine by means of an aqueous surfactant producing a foam having a relatively high viscosity and a correspondingly reduced mobility. According to Wellington et al., carbon dioxide foam flooding is often a more effective alternative to carbon dioxide gas flooding for the displacement of oil from a subterranean formation.
The effectiveness of carbon dioxide foam flooding is further enhanced by the incorporation of viscosifiers into the foam, such as water-soluble polymers disclosed in U.S. Pat. No. 5,129,457 to Sydansk. The resulting polymer enhanced foams have been shown to outperform their polymer-free counterparts in low-pressure oil recovery applications within fractured formations, apparently due to the increased viscosity and stability of the polymer enhanced foam.
Another approach to improving the effectiveness, and in particular to decreasing the mobility of gas floods, is to place a more viscous polymer/surfactant solution in a homogeneous formation ahead of a miscible gas flood as taught by U.S. Pat. No. 4,676,316 to Mitchell. The polymer/surfactant solution effects simultaneous mobility and profile control of the subsequent miscible gas flood as evidenced by delayed gas breakthrough. Consequently, the process of Mitchell increases sweep efficiency, maintains gas flow frontal stability and avoids bypassing of oil caused by formation permeability stratification.
Yet another approach to improving the effectiveness of gas floods is taught by U.S. Pat. No. 4,609,043 to Cullick. A carbon dioxide gas flood is viscosified by condensing the gaseous carbon dioxide under supercritical conditions and dissolving a water insoluble polymer directly in the supercritical carbon dioxide with the use of a polar organic solubility agent, such as a monohydric lower alcohol or a glycol. Carbon dioxide in a supercritical state has been shown to be a particularly effective oil displacement fluid relative to carbon dioxide in the gaseous state.
A process whereby a viscosifier is dissolved or otherwise dispersed within the supercritical carbon dioxide without the use of a solubility agent, however, would be a preferred alternative to the process of Cullick from both an economic and operational standpoint. Yet, as noted in the Wellington et al. reference, carbon dioxide viscosifiers have been studied for years without success due to their low solubility in carbon dioxide.
Accordingly, it is an object of the present invention to provide a liquid hydrocarbon recovery process utilizing a displacement fluid having favorable stability, viscosity and mobility characteristics as well as favorable oil displacement characteristics. It is another object of the present invention to provide such a process whereby the bulk of the displacement fluid is made up of relatively inexpensive and plentiful components. It is yet another object of the present invention to provide a liquid hydrocarbon recovery process whereby the displacement fluid has a viscosifier phase readily dispersible within a carbon dioxide phase in the absence of a solubility agent.
It is an alternate object of the present invention to employ the above-described fluid as a mobility control fluid in conjunction with a liquid hydrocarbon displacement flood. It is another alternate object of the present invention to employ the above-described fluid as a permeability-reducing fluid or blocking agent in a conformance improvement treatment associated with a liquid hydrocarbon displacement flood.